Re: Cap question(s)

To: tesla-at-pupman-dot-com
Subject: Re: Cap question(s)
From: Bert Hickman <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Thu, 15 Jul 1999 07:01:49 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Tesla List wrote:
> 
> Original Poster: Gavin Hubbard <ghub005-at-xtra.co.nz>
> 
> Hello list.
> 
> I am building an 8" coil with a 11kV, 5kVA pole transformer. I have
> finished most of the work but haven't sorted out the primary capacitor for
> this system.
> 
> I have 20 new 0.1uF, 5kV glassmike caps which I planned to use for this
> project, but I have been offered (for money) a good pair of used 0.06uF,
> 40kVDC Maxwell pulse caps. What, if any, difference in performance should I
> expect between similarly configured caps in an otherwise identical system?
> 
> Of course the glassmikes will be slightly lossy and probably get warm
> during a run, and the Maxwells would have a lower ESL and ESR, but will
> this alter the real world performance of the coil?
> 
> Cheers
> 
> Gavin Hubbard
> 
> P.S. When designing a reso cap system, many people simply calculate the
> impedance of the NST from output V and I, then assume its reactive and
> build a matched cap. I've always wondered what the actual impedance of the
> NST is...surely with hundreds of metres of wire in the secondary it must
> have a large resistive component?

Gavin,

Go for the Maxwell's! Glassmikes are intended for DC use, with minimal
AC ripple current. Most use Mylar dielectric system, which is more than
a _little_ lossy at RF frequencies. Your Glassmikes will not only get
very warm very quickly, there's a good chance that one or more may
explode, since the cylindical housing is very unforgiving to internal
heating and build-up of pressure. While there are a few glass-enclosed
caps that use polypropylene dielectric (Blue-colored type HG caps from
Plastic Capacitors for example), even these are not intended for the
electrical and thermal stresses seen in Tesla Coil tank capacitor
service. 

Maxwell polypropylene (PP) pulse caps are superb for Tesla Coil use, and
really come into their own in high power systems. The 0.06 uF 40 kV PP
variety (such as Model #31393, or the newer model #440PD860) are very
commonly used in pulsed laser systems... and by Tesla Coilers lucky
enough to find them on the surplus market, or wealthy enough to buy them
new (Major OUCH!). Their rectangular plastic housing permits a bit of
expansion to occur under heavy duty use, and they are built to take
substantial punishment. The polypropylene dielectric system will give
you unexcelled performance - there is literally NO comparison between
Maxwell pulse caps and the Glassmikes...

Safe coilin' to you, Gavin!

-- Bert --